Coal combustion ash (CCA) is a by-product derived from the combustion of coal and comprises finely divided inorganic products. Enormous amounts of CCA are produced annually worldwide, primarily from burning coal in electric power plants. Disposal of CCA has posed an increasing difficult problem because the volume increases annually, because of the light and dusty nature of the material, and because the varied chemical composition of CCA limits the number of acceptable disposal methods and sites.
Efforts have been made to find alternative economic uses for CCA. For example, CCA has been used as an additive in Portland cement. However, the fraction of CCA which is suitable for use in concrete represents a small portion of the total amount of ash available. Another use for CCA waste is as fill in asphalt and land reclamation. These uses have low economic value and the risk of heavy metals leeching out of the CCA is a concern. The world needs more economically advantageous uses for CCA to reduce the burden and recoup value. One promising use is as a raw material for ceramic articles.
Traditional Ceramics Formulations and Formation Methods
Ceramic articles are typically made by forming wet clay into desired shapes followed by active drying. Sometimes these shapes are formed by hand, but more commonly they are pressed into shape by machine. This is particularly true of ceramic tiles and other stoneware. The dried article has sufficient dimensional stability and strength to withstand handling. The article is then fired in a kiln to 1250-1350 degrees C. to fuse the clay particles together. The material is sufficiently heated to cause the clay particles to melt and fuse into a glassy mass. This process is called vitrification and the material must be cooled slowly to avoid cracking. The liquefaction of the material ensures that the pores between particles have all disappeared and the water absorbance is low, but this requires high temperatures and therefore has high energy costs.
Alternative Ceramics Formulations and Methods
Bisque Sintering
A related method for forming ceramic articles is called bisque firing. In this process, a shaped article is dried to gain strength, and then the dry article is heated to roughly half of its melting temperature. Compacted powders will begin to fuse when heated to about half of their absolute melting temperature. This produces “bisque-ware” and a common trait of these articles is that the surface area of the article decreases (the article shrinks) while the strength of the fired article increases. In bisque-ware, clay particles partially fuse without completely vitrifying. The article is firm and will no longer soften in water, but is very porous and not as strong as it will be after final firing. The material can be handled carefully at this stage, however, after firing the material must be cooled slowly to avoid cracking.
Glaze material is then applied to the surface of the article and is fired again at higher temperatures. The glaze melts and adheres to the surface of the article and the body composition of the article resumes sintering to a point where the material is almost completely vitrified and forms a solid ceramic material. The glaze coating not only adds decoration but also seals the article to achieve very low water absorbance. FIG. 1 schematically shows the method for forming conventional bisque-ware. The process entails forming a partially fused material with many pores, applying a glaze to the article, and re-firing to seal the bulk material.
Liquid Phase Sintering
Another related method to the traditional methods of making clay articles is to perform liquid phase sintering where a low-melting material (the additive) is introduced into the clay mixture. This material melts at a temperature below the sintering temperature of the clay particles (base particles). The melted liquid phase pulls together the clay particles through capillary action into a close-packed arrangement that minimizes the space between the solid particles. This process is driven by the thermodynamics of the system to reduce the wetted area of the liquid. As the temperature continues to rise, the close-packed clay particles melt together expelling the low-melting (now liquid) material. For the details, see “Liquid Phase Sintering”, Randall M. German, Plenum Press, 1985. This process requires high temperatures and there is a risk of product deformation (slumping) when the second composition melts and the article loses dimensional stability during heating. Often, molds are used to preserve the shape of an article. Also, the material must be cooled slowly to avoid cracking. FIG. 2 schematically shows the method of conventional liquid phase sintering. As can be seen, first the melt phase forms a liquid and draws the clay particles together. Then, the clay particles melt to form a fused block of material whereupon significant shrinkage occurs.
The traditional liquid phase sintering of clay involves selecting a clay/additive combination where the additive has a high solubility for the clay material, but the clay material has a low solubility for the additive. In this way, the melted additive is semi-permanent liquid during the process; it is not absorbed into the clay base material. As the temperature is raised and the additive melts, it does not soak into, or dissolve into the clay, but instead draws the clay particles together through capillary forces that serve to reduce the volume of the liquefied additive. The temperature is raised until the clay particles begin to fuse. Since the clay particles are close together due to the capillary action of the melted additive, they fuse slightly (forming a “neck” between particles) and then eventually they fuse together completely forcing out the liquefied additive from between them. Any voids that remain are filled with the melted additive.
Limitations of the liquid phase sintering are that the article can slump when the melting additive begins to melt. This is because there is no structural framework throughout the clay material to support the article between the time the additive melts and the time the clay material begins to fuse. Additionally, the fusing temperature range is narrow and can be easily exceeded. When this happens the material does not fuse together but instead completely melts and all shape is lost as the material liquefies. Another problem is that the lower temperature melt additive material can run out of the bulk material if the article is not constrained in a mold.
Tile Formation
One type of article that can be produced using these methods is ceramic tiles for floors or walls. They are usually decorated or undecorated flat panels that fit together to form a pattern on a wall or floor. Traditional methods of tile formation involve mixing clay and other minerals together to form dough which can be formed into shapes by pressing or forming in other ways, and then fired. Usually the ingredients are mixed in a ball mill with a fixed amount of water. The average particle size is reduced in ball mills and causes all the ingredients to be intimately mixed. The mixture leaves the ball mill as a slurry which is spray-dried to remove the water and to produce a granular material that is used in further processing. The granular material forms a free-flowing powder that is easy to transport and fills the molds used in ceramic tile die cavities.
Ceramic tile dies are filled with this granular mixture and then hydraulic presses form tiles in the cavities. The tiles are dried and glaze is applied for decoration and to seal the tile (providing low water absorbance). The glazed surface also makes the tile hard and provides a durable surface that resists wear. The decorated tile typically then goes into a tunnel kiln where it is fried to sinter the tile and melt the glaze into the surface layer. During the firing the clay and other ingredients fuse together to form a solid ceramic material.
Some methods for forming ceramic articles use coal ash or CCA or other waste as raw materials, for example, in GB1058615, U.S. Pat. No. 3,679,441, U.S. Pat. No. 5,521,132, U.S. Pat. No. 6,743,383, U.S. Pat. No. 5,935,885, U.S. Pat. No. 6,566,290, AU708171, WO03059820, as well as CN1260336, CN1410386, CN1029308, CN101372414, CN1070177, etc.
U.S. Pat. No. 5,935,885 discloses ceramic tiles made from CCA and other incinerator wastes but the methods used involve oxidation of the mixture components at a temperature of 1000 to 1500 C and high temperature vitrification (1250 to 1550° C.) to completely melt all the aluminosilicates with the goal of locking in heavy metal contaminates.
U.S. Pat. No. 6,743,383 discloses using industrial wastes, including CCA, to make ceramic tiles, but only use low percentages of wastes.
U.S. Pat. No. 5,521,132 disclose a ceramic materials made from coal fly and municipal solid waste CCA and processes for producing the ceramic materials, in which over 85% weight percent of CCA is included in the ceramic material and is bonded together by water-insoluble reaction products produced by the reactions between the components of a molten flux and portions of the residue CCA particles which may have dissolved into the molten flux. This process uses sodium tetraborate as an additive and includes the step of firing at a temperature of from just above the melting point of sodium tetraborate to about 1000 degree C.
The above processes suffer from several disadvantages: the waste material especially the CCA needs to be melted at a higher temperature thereby significantly increasing energy consumption; the cooling of ceramic article lasts for a long time, leading to the consumption of more energy; and some of them require the use of a mold. Therefore, it is desirable to provide an improved process for forming aluminosilicate articles such as ceramic articles that uses less energy, lowers the cost and provides the ceramic article having improved properties such as higher strength and lower water absorption.